In their chemical behaviour, the earlier members of the lanthanoid series are quite reactive, similar to calcium. However, with increasing atomic number, their reactivity decreases and they behave more like aluminium. Thus, the statement that all lanthanons are much more reactive than aluminium is incorrect.

The other statements are correct:

• Europium ($\text{Eu}$) can exhibit a $+2$ oxidation state due to the stability of its half-filled $f^7$ configuration.
• Due to lanthanoid contraction, the ionic radius of $\text{Ln}^{3+}$ ions decreases steadily from $\text{La}$ to $\text{Lu}$. This decrease in size increases the covalent character of the $\text{Ln}-\text{OH}$ bond, thereby decreasing the basicity of their hydroxides.
• $\text{Ce}^{4+}$ is a strong oxidising agent because it tends to accept an electron to revert to the more stable $+3$ oxidation state. It is widely used as a standard oxidant in volumetric analysis.